Surface Morphology versus Chemical Modification - American

Aug 30, 2007 - recordable compact disc (CD-R) gold substrates can be selectively modified ... SAMs in parallel “microstrips” that are analogous to...
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J. Phys. Chem. C 2007, 111, 14157-14164

14157

Monolayer-Directed Electrodeposition of Oxide Thin Films: Surface Morphology versus Chemical Modification Dinah M. Soolaman and Hua-Zhong Yu* Department of Chemistry, Simon Fraser UniVersity, Burnaby, British Columbia V5A 1S6, Canada ReceiVed: February 14, 2007; In Final Form: June 26, 2007

In this paper we describe the general role that surface morphology and chemical nature play in guiding the cathodic deposition of oxide thin films onto electrode surfaces. By use of a “featureless” stamp for microcontact printing (µCP), the pregrooved microfeatures (“mountains” and “valleys” representing the track trails) of recordable compact disc (CD-R) gold substrates can be selectively modified with OH- or CH3-terminated self-assembled monolayers (SAMs). For comparison, “flat” gold substrates were patterned with the above SAMs in parallel “microstrips” that are analogous to the CD-R substrate (but no height differences). Electrochemical deposition of zirconia thin films showed that, on the CD-R substrates, surface morphology (height difference) dominates over the blocking effects of the SAMs; that is, deposition occurred primarily on the mountains despite these sites being modified with organic monolayers. For flat gold substrates it was found that n-alkanethiolate SAMs block deposition in modified areas while directing the deposition to regions of the bare surface. When flat gold substrates were modified with CH3- and OH-terminated SAMs in alternating microstrips, deposition was confined to “narrower” regions that are different from the periodicity on the stamp. The type of microstructures and feature sizes of the zirconia thin film were dependent on scan rate, number of cycles, and terminal groups of the SAM to a lesser extent.

Introduction Nanofabrication has generated a great deal of interest in recent years, with studies focused on various methods to fabricate micro- and nanostructures on diverse substrates. Control of the patterning and uniform deposition of metal oxide films is key to the development of technologically useful structures and devices.1 In the past, electrodeposition has been proven to be a simple method to prepare microstructures on conducting substrates; it is a rapidly growing field for making metal oxide films via electrochemical depostion.2-4 This technique has several advantages: (1) uniform films can be formed on substrates of complex shapes; (2) the film morphology and thickness can be controlled by electrochemical parameters; and (3) the equipment cost is low.5 Selectivity of film deposition can be achieved by exploiting either the chemical nature or surface morphology of the substrate, with the potential of making electronic devices of diminishing scale.6 Substrates have been patterned on the microand nanoscale by a variety of methods such as photolithography (UV irradiation), photocatalytic lithography, microcontact printing (µCP), focused ion beam bombardment, and chemical etching.8-13 Among these, µCP has been very successful, as it is a relatively simple technique that can be done in a regular laboratory setting.14-18 On various conductive substrates such as gold, self-assembled monolayers (SAMs) of n-alkanethiols and their derivatives have been used to control film deposition.19-21 Typically, SAMs are used as resists to protect gold surfaces from oxidative etching or to block certain reactions.6 In electrodeposition, these monolayers may be used to passivate the electrode surface, directing deposition to proceed on bare regions. Seo and Borguet22 controlled deposition to small * To whom correspondence should be addressed: phone (778) 7828062; fax (778) 782-3765; e-mail [email protected].

patterned areas by using an atomic force microscopy (AFM) tip to “shave” away portions of a 1-hexadecanethiolate SAM on gold. This allowed “nanotemplates” to be formed with very low lateral dimensions since the highly ordered SAMs block deposition, limiting it to the exposed regions. Zhou et al.6 used patterned SAMs prepared by µCP as templates to direct the deposition of polypyrrole microstructures on gold and silicon. Tao et al.7 found that certain polymer films could be patterned on a gold surface by use of SAMs as the resist, and they compared the effect that chain length and terminal groups have on passivation of the surface. Similarly, the morphologies of electrode surfaces have been used to control the film depositions previously. Walter et al.2 showed that it is possible to selectively deposit metals (Mo) and metal oxides (MoO2) at the step edge of a highly oriented pyrolytic graphite surface. It was found that MoO2 nucleates exclusively at step edges of the graphite surface, and this selectivity was evident without chemical treatment of the step edges. In our group, electrodeposition of zirconia thin films was previously carried out on gold recordable compact disc (CDR) substrates;23 these surfaces have a unique pregrooved topography that consists of uniform “mountain-valley” stripes at the micrometer level. Upon modification of the entire substrate with different SAMs having various terminal groups (such as COOH, OH, and CH3), it was observed that the SAMs exhibited notable control over the formation and morphology of the microstructures. In retrospect, electrochemical deposition of zirconia thin films has been well studied,3,4,24-30 and they are known to have practical applications as solid-state electrolytes for fuel cells, oxygen sensors, and catalysts. To augment the significant potential for mass production of patterned microstructures, it is important to understand the influence that both the surface morphology and molecular templates have in guiding electrodeposition.

10.1021/jp071290+ CCC: $37.00 © 2007 American Chemical Society Published on Web 08/30/2007

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Figure 1. The four systems studied on gold CD-R substrate with a prepatterned “mountain/valley” morphology: (A) the mountains modified with C10 SAMs, (B) C11OH SAMs on the mountains, (C) C10 SAMs on the mountains and C11OH SAMs in the valley, and (D) C11OH on the mountains and C10 SAM in the valleys.

This work aims at a better understanding of the role that surface morphology versus chemical nature plays in directing the deposition of oxide thin films, by comparing “flat” gold substrates patterned with alkanethiolate SAMs to that of gold CD-R substrates (with mountain and valley features). In particular, CD-R substrates are used in conjunction with featureless PDMS [poly(dimethylsiloxane)] stamps to prepare SAMs on the mountains, whereas another deposition solution will be used to modify the valleys (Figure 1). Such a unique approach produces a micropatterned substrate that has specific regions chemically tailored with either OH or CH3 groups and also different in heights. In comparison, “flat” gold substrates were patterned with microstrips of OH- or CH3-terminated SAMs. Subsequent electrodeposition of zirconia thin films onto these substrates was conducted to explore the resulting material microstructures when substrates possess varied patterns and molecular termini. Experimental Section Materials. 11-Mercapto-1-undecanol (97%), 6-mercapto-1hexanol (97%), 1-octadecanethiol (98%), and 1-decanethiol (96%) were purchased from Aldrich (Milwaukee, WI). HNO3 (70%) and ethanol (95%) were of ACS reagent grade and were used without further purification. Deionized water was obtained from a Barnstead EasyPure UV/UF compact water system (Dubuque, IA) with a resistance of 18.3 MΩ·cm. Gold CD-Rs

Soolaman and Yu without logos (MAM-A Mitsui CD-R Gold) were purchased locally. “Flat” gold substrates (regular glass slides first coated with 10 nm of Cr, followed by 100 nm of Au) were purchased from Evaporated Metal Films (EMF) Inc. (Ithaca, NY). We note that these gold surfaces are not atomically flat; the term “flat” is merely used to compare with CD-R substrates that have mountain/valley features at the micrometer scale. Poly(dimethylsiloxane) (PDMS) stamps were made by curing a 10:1 mixture of elastomer and hardener (Dow Corning Corp., Midland, MI). To obtain flat stamps, the elastomer was cured over a Petri dish that was spin-coated with a thin layer of the precursors. Stamps patterned with 20-µm strips were obtained by curing the elastomer over lithographic silicon master templates. The PDMS stamps were then cut into convenient sizes for µCP experiments. Sample Preparation. CD-Rs were cut into suitable sized slides and treated with concentrated HNO3 for 5 min to remove the protective polymer layer, rinsed with copious amounts of water, and dried under N2. “Inking” solutions for the PDMS stamps were prepared from 1.0 mM 11-mercapto-1-undecanol or 1-decanethiol in 95% ethanol. The PDMS stamp was then inked with the solution by use of a Q-tip and dried under a stream of N2. For modification of the mountains of the CD-R, the flat PDMS stamp was pressed onto the surface for ∼30 s. To subsequently modify the valleys, these substrates were then immersed in solution of 1.0 mM of 11-mercapto-1-undecanol or 1-decanethiol in 95% ethanol for about 30 min. These substrates were then removed, rinsed in 95% ethanol and water, and dried under N2. “Flat” gold slides were treated in a “piranha” solution (3:1 mixture of concentrated H2SO4 and 30% H2O2) for 5 min at 90 °C to remove organic contaminants (CAUTION: use extreme care as piranha is explosiVe when in contact with organic materials). Patterned PDMS stamps (20 µm strips) were inked with 1.0 mM 1-decanethiol or 1-octadecanethiol solutions and stamped onto the substrate for ∼30 s. Patterns of squares (grids) were obtained by stamping the substrate twice with the strips perpendicular to each other. To create substrates with both CH3- and OH-terminated SAMs, the patterned substrates were subsequently treated in another solution, 1.0 mM 6-mercapto1-hexanol or 11-mercapto-1-undecanol for 30 min, rinsed in 95% ethanol, and dried under N2. Modification of the gold surfaces with SAMs was confirmed with wetting measurements. Instrumentation. Electrodeposition was performed by use of an Autolab electrochemical analyzer (PGSTAT30, Eco Chemie BV, Netherlands) in a Faraday cage. The working electrode was the gold substrate, the counter electrode was a Pt wire, and the reference electrode was a Ag|AgCl|3 M NaCl. The gold chip was pressed against the bottom of a Teflon cell with an O-ring seal, exposing an area of 0.69 cm2 on which deposition takes place. Solutions for the electrodeposition of the metal oxide consisted of 5.0 mM ZrOCl2‚8H2O and 0.10 M KCl in deionized water. Zirconia films were formed by cycling at various scan rates (from 20 to 200 mV/s) for typically 5-15 cycles from +0.8 to -1.1 V (vs Ag|AgCl). Atomic force microscopy (AFM) images were obtained from a Topometrix Explorer AFM (8 µm Z-linearized scanner) with a silicon nitride tip (Triangular D of MSCT-AUHW, Veeco Metrology group, resonance frequency 15 kHz, force constant 0.03 N/m). Images were obtained in contact mode and analyzed by use of Thermomicroscopes SPM Lab Software. Scanning electron microscopy (SEM) images were obtained from a FEI DualBeam Strata 235 system (Hillsboro, OR). Samples were imaged via secondary electron imaging (SEI) and grounded by use of carbon paste to avert charging of the sample.

Electrodeposition of Oxide Thin Films

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Typical accelerating voltages of 10 kV and magnification of ×5000 was used. Energy-dispersive X-ray (EDX) analysis was also performed to obtain elemental analysis of the zirconia film formed. Results CD-R Substrate. Four systems were studied on the gold CD-R substrate (Figure 1): (A) 1-decanethiolate monolayer (C10) on the mountains; (B) monolayer of 11-mercapto-1undecanols (C11OH) on the mountains; (C) C10 on the mountains and C11OH in the valley; and (D) C11OH on the mountains and C10 in the valleys. Subsequently, zirconia thin films were electrochemically deposited onto the surface; that is, multiple cyclic voltammetric (CV) scans were performed with these modified gold substrates as working electrodes in the presence of 5.0 mM ZrOCl2 (0.1 M KCl as the supporting electrolyte) as the precursors. In the system where the mountains of the CD substrate were modified, it was found that the SAMs serve to direct the deposition of zirconia thin films on the surface. SEM was used to study the confined deposition of zirconia thin films on CD substrates, because AFM imaging of these surfaces having both microscale (due to the underlying CD morphology) and nanoscale roughness (the electrochemically deposited oxide film) proved very challenging. As shown in Figure 2, electrochemical deposition occurred on the mountains while there was little or no deposition observed in the valleys. This effect was more pronounced for the C10 system than for other systems studied. These results are surprising, as it is generally believed that hydrophobic SAMs (CH3-terminated) exhibit a blocking effect and would inhibit nucleation, thus preventing deposition at these locations. However, confinement is almost exclusively observed on the mountains in our experiments despite the fact they are covered with CH3-terminated SAMs. Furthermore, it was found that the potential scan rate significantly influences the selective deposition of the oxide thin films on the modified CD-R substrate (Figure 2). At slower scan rates, even though there was pronounced deposition on the mountains, deposition also occurred in the valleys. At faster scan rates of 50-100 mV/s, deposition of zirconia was almost completely confined to the mountains with smaller particles observed. As the scan rate decreases to about 50 mV/s, the size of the oxide particles that make up the film begins to increase. Although the deposition is still presumably confined to the mountains, due to the larger grains, the zirconia begins to spread into the valleys. At even slower scan rates (20 mV/s and below), deposition occurs both in the valleys and on the mountains. It has been found that varying the number of scans also influences the deposition. At slower scan rates (10), deposition occurs both in the valleys and on the mountains. For faster scan rates (50100 mV/s), a larger number scans (up to 15) can be used while still having deposition isolated to the mountains (Figure 2). However, as the number of scans increases, deposition begins to occur in the valley also. Therefore, it can be concluded that the monolayer patterned on the surface controls only where the initial deposition occurs. As more scans are performed on the electrode, both the bare areas and the areas covered with monolayers will have zirconia film deposition. When the mountains were modified with OH-terminated SAMs (C11OH), confinement of the film was not as readily observed as in the previous system (C10). At a scan rate of 20 mV/s running for 10 cycles, there was no preferential deposition observed but rather large random agglomerates of

Figure 2. Electrochemical deposition of zirconia thin films on gold CD-R substrates where mountains were modified with C10 SAMs. These SEM images were obtained for samples prepared under different electrochemical conditions: (A) 15 scans at 30 mV/s, (B) 15 scans at 50 mV/s, and (C) 15 scans at 200 mV/s. (Inset) SEM image of gold CD-R substrates before deposition.

zirconia formed on the entire surface (Figure 3). At scan rates between 30 and 50 mV/s (10 cycles), deposition became pronounced on the mountains but was still observed in the valleys. It is apparent that the OH-terminated monolayers are not as efficient as expected on directing the deposition of the zirconia films. At much faster scan rates (relative to C10 on mountains systems) (e.g., 100 mV/s) run for 15 cycles, preferential deposition was observed only on the mountains, and smaller zirconia particles were observed. Generally speaking, confinement of the zirconia film on mountains modified with OH-terminated monolayers was not as easily obtained except at very high scan rates. This indicates that although the mountains favor deposition, the chemical modification with

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Figure 4. Electrodeposition of zirconia thin films on gold CD-R substrates where both the mountains and valleys were modified with SAMs. The SEM images correspond to deposition on CD-R substrates patterned with (A) C10 SAMs on the mountains and C11OH SAMs in the valleys (10 scans at 20 mV/s) and (B) C11OH SAMs on the mountains and C10 SAM in the valleys (10 scans at 20 mV/s), respectively.

Figure 3. Electrochemical deposition of zirconia thin films on gold CD-R substrates where mountains were modified with C11OH SAMs. The SEM images are obtained for samples that were prepared under different electrochemical conditions: (A) 10 scans at 20 mV/s, (B) 10 scans at 30 mV/s, and (C) 15 scans at 100 mV/s.

different terminal groups play a role (to a lesser extent) in guiding the deposition. When the mountains and valleys of a gold CD-R substrate were modified with different SAMs, no preferential deposition was found as indicated by the SEM images (Figure 4). A thin film was generated on both the valley and mountain regions and cracks appeared throughout the surface, regardless of the choice of terminal groups. The mountain regions were covered with larger amalgamates of zirconia such that crossover occurred into the valley region. It is believed that initially a thin uniform layer is first deposited over both valley and mountain. Subsequently, building-up and growth of the larger zirconium particles occur on the mountains and they eventually cross over from mountain to mountain and into the valley. There was little difference in the film morphology prepared at 20 mV/s

compared to 100 mV/s, and confinement of the zirconia film was not remarkable in these systems. “Flat” Gold Substrate. Three systems were studied: (1) CH3-terminated SAMs patterned in 20-µm stripes with alternating areas of bare gold, (2) CH3-terminated SAMs patterned in grids with squares of bare gold, and (3) CH3-terminated SAMs patterned in 20-µm stripes with alternating OH-terminated SAMs. Gold slides patterned with 1-octadecanethiolate (C18) SAMs showed a notable effect to control the deposition of zirconia thin films when optimal electrochemical parameters were used (in comparison, gold surface patterned with C10 did not). For each substrate, 10 scans were performed, ranging from +0.8 to -1.1 V at 20, 30, 40, 50, and 100 mV/s. Scanning to potentials more negative than -1.1 V resulted in severe hydrogen evolution (a number of bubbles being formed) and yielded nonuniform films, as selectivity of the electrodeposition is lost. Also, thinner films showed better confinement, which was best observed scanning from +0.8 to -1.1 V at 40 mV/s. These results indicated that the SAMs serve as excellent resists to guide deposition to regions of bare gold. The zirconia films obtained with this method were very uniform, showing that fine control could be obtained for building nanostructures by use of electrochemical deposition (Figure 5). Due to these films being significantly less rough than those formed on the CD-R substrate, they were imaged with AFM to examine their microscopic topography. It was evident that the formation of zirconia thin films can be confined to strips of arbitrary width

Electrodeposition of Oxide Thin Films

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Figure 5. Electrochemical deposition of zirconia thin films on “flat” gold substrates patterned with C18 SAM in 20-µm alternating strips (with bare gold). The AFM image shows the selective deposition of zirconia films (10 scans at 100 mV/s) onto regions of bare gold. The cross section profile below indicates the film thickness in regions of preferential deposition to be approximately 150-200 nm.

(depending on the dimensions of the PDMS stamps used) having uniform heights of a large range (depending on the number of cycles performed). Figure 5 shows the structure obtained from patterning C18 SAMs in strips of 20-µm width having zirconia thin film deposition in thickness of approximately 150 to 200 nm. Although slight deposition also occurred in regions covered by C18, it is remarkable that electrochemical deposition of zirconia is highly favored in the regions of bare gold. To show that this approach can be employed to confine deposition to regions of varying shapes and sizes, we patterned “microsquares” on the surface. This was done by applying the patterned PDMS stamp twice on the substrate in perpendicular directions to create squares of bare gold. Electrodeposition performed on these surfaces showed clear confinement, yielding square patterns of uniform films (Figure 6). Topographic data obtained from AFM studies shows that the squares of zirconia films have heights of 220 ( 20 nm. SEM images further confirm the notable confinement and uniformity of the zirconia films on the microsquares of bare gold in a larger scale (Figure 6B). This is evident from the contrast of the image; regions with substantial film buildup appear darker due to these regions being less conductive, whereas regions that lack buildup of the metal oxide film appear lighter since these regions are more conductive. Electrochemical deposition of zirconia thin films was also carried out on gold substrates consisting of alternating 20-µm stripes of CH3- and OH-terminated SAMs. AFM analysis showed no obvious confinement in the C11OH/C18, C11OH/ C10, or C6OH/C10 systems since the entire cathode surface was now covered with inert SAMs. Remarkably, in the C6OH/ C18 system, the deposition showed substantial confinement (Figure 7): microstrips of zirconia thin films of about 10-µm width and 400-nm height were formed, for which the periodicity is different from the PDMS stamp (20-µm wide strips). Similarly to other systems, slight deposition was also observed over other areas of the surface.

Figure 6. Electrochemical deposition of zirconia thin films on “flat” gold substrates patterned with microgrids of C18 SAMs (leaving squares of bare gold). (A) AFM image and a cross section profile; (B) SEM image. Electrochemical deposition was carried out in 5.0 mM ZrOCl2/ 0.1 M KCl for 10 scans at 40 mV/s.

Discussion Our results indicate that not only surface morphology but also chemical modification play a role in guiding the deposition of metal oxide films under electrochemical conditions. The method of cathodic deposition that was used here relies on the generation of base (OH-) at the electrode surface (gold) by the following reaction: 2H2O + 2e- f H2 + 2OH-.3 Subsequently, the ZrO2+ cations migrate toward the cathode and are hydrolyzed by the base to form the hydroxide complexes. This is followed by dehydration and condensation to yield the metal oxide (ZrO2) when the electrode is removed from the electrolyte.3,30 As a result, alkanethiolate SAMs can be used to passivate regions of the cathode surface, thereby blocking deposition and inhibiting film formation to selected areas. The CVs for the zirconia deposition show an irreversible redox process (Figure 8a), with the cathodic peaks around -0.95 and -1.05 V believed to be due to the complex redox behavior of ZrOCl2 on gold.23 As deposition proceeds, with each additional cycle the reduction peaks are less distinct and the peak current decreases (as shown in Figure 8a) as a result of the growing film being resistant to the redox process. This is also indicated by the decreased charge when the number of scans increases (Figure 8b).

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Figure 7. Electrochemical deposition of zirconia thin films on “flat” gold substrates patterned with C18 and C6OH SAMs in alternating strips of 20-µm width. The AFM image shows the confined deposition of zirconia films as narrower microstrips (15 scans at 40 mV/s).

The regions modified with organic monolayers should exhibit a blocking effect on the electrode surface to prevent electrodeposition at these locations. However, on the CD-R substrates, it was found that edge effects (or height difference) from the topography dominate over the blocking effects from the alkanethiolate SAMs. Although edge effects have been noted in the literature, there is still much to be learned in understanding this phenomenon. By comparing the effect that height differences have in dictating deposition, we have gained some insight. In these studies it becomes evident that at higher scan rates the deposition occurs predominantly on the mountains irrespective of the chemical modification of the surface. This may be due to the depletion of ZrO2+ in the “valleys” at faster scan rates (as the surface concentration of the redox cations is dictated by the potential sweep rate and influenced by the electrode morphology), such that little or no deposition is observed. Previous studies deduced a strong correlation between the local concentration of cathodically generated base (OH-), concentration of ZrO2+, and the microscale mountain-valley surface profile on the selectivity of zirconia thin films prepared by electrodeposition.23 The preferred coagulation of zirconia particles on the mountain regions may also be due to stronger electric fields along the edges in addition to ionic depletion in the valley.23,31 It is known that electric fields are stronger near small radii of curvature; therefore, along the edges of the micropatterned CD-R substrate this high field works to confine the film to the mountains and dominates over chemical modification with monolayers.32-33 As the scan rate decreases, it is believed there is more time for the electrolytes to diffuse to the valley; therefore, film formation is observed both in the valleys and on the mountains. As the number of scans increases for both high and low scan rates, a lot more crossover has been observed with deposition. It is evident that surface morphology plays the dominant role and the deposition occurs selectively in mountain areas, as the modification with CH3-terminated monolayers (C10) produced even better results for film confinement (Figure 2). In their study on electrochemical deposition on top of organic monolayers,

Figure 8. (a) CVs of electrochemical deposition of zirconia thin films onto CD-R gold substrate modified with C10 SAMs on the mountains at 30 mV/s. (b) Integrated charge as a function of the scan numbers. As the film builds up, the current (as well as the charge) decreases as the substrate becomes increasingly resistant to the redox reactions; therefore, the growth of the film reaches its limit.

Qu and Uosaki34 noted that defect sites in SAMs play a significant role, that is, they act as nucleation centers since the large amounts of free metal ions in the bulk solution can penetrate the organic layer via defect sites where deposition is initiated. The C10 monolayer is not closely packed (compared with long-chain ones, e.g., C18) and is more prone to electrolyte penetration and reductive desorption processes. When the valley was not modified, base generation occurs more readily at these sites, creating a hydrophilic local environment relative to the hydrophobic mountain regions (covered with C10 SAMs). It is believed that the interface along the edge between hydrophilic valleys and hydrophobic mountain further contribute to enhanced edge effect, favoring a high nucleation density in these areas. Thus initial nucleation is favored at defect sites along the mountains, as there is a higher flux of particles to this region; therefore, the film continues to grow in these regions with very little deposition occurring in the valleys as the concentration of ZrO2+ is depleted in these regions. When the mountains were modified with C11OH monolayers, selective deposition was not as evident: slower scan rates resulted in deposition over both the valley and mountains. It

Electrodeposition of Oxide Thin Films was not until very high scan rates, >100 mV/s, that confinement of zirconia thin film deposition was observed on the mountains. We believe that this is because the functional groups at the monolayer surface also play a role (though to a lesser extent) in guiding the deposition.23 The surface charge is sensitive to the local pH and affects the film formation, which has been shown in studies with acidic groups.35,36 In this system the local environments are hydrophilic in the unmodified valley (due to cathodic base generation) and on the mountains (OH-terminal SAMs). Base generated in the valleys can migrate to accommodate electrochemical reactions along the mountains, but deposition can similarly occur in the valleys, as there is no enhancement of edge effects due to differences of local environment caused by molecular modification. Rather, deposition proceeds in the valley and at defects within the C11OH SAM on the mountains. At high scan rates, the effect of the chemical groups become less significant, and edge effects once again begin to dominate, which is seen by pronounced deposition on mountains at 100 mV/s. In the case when both the mountain and valley are modified with monolayers (different terminal groups), selective deposition has not been clearly observed. This may be attributed to the entire cathode surface being blocked by organic monolayers, hindering the generation of base. The random film formation is likely due to nucleation at defects within the monolayers (created either by the µCP process or by potential scanning to such negative values, which may remove portions of the monolayer, creating pinholes), which act as microelectrodes for electrochemical deposition.33,37 Although the local environment between the valley and mountain is hydrophilic and hydrophobic, such that it is presumed an enhanced field should exist along the edge, cathodic generation of base is severely hindered since the SAM blocks the entire cathode. Therefore, sites where defects exist in the monolayer will generate base and initiate nucleation. Flat gold substrates patterned with CH3-terminated monolayers (C18 in stripes) with alternating regions of bare gold showed notable confinement. Analysis with AFM and SEM showed that there is clear confinement of the zirconia thin film onto the regions of bare gold. In addition, EDX analysis shows the increased signal of zirconia on regions of bare gold (data not shown). This analysis confirms that deposition of the zirconia film is favored in regions of bare gold; however, a thin layer of zirconia is also deposited on the regions covered with monolayers. Consistent with previous findings,6 we found that the C18 monolayer had much better blocking effects than shorter-chain SAMs (e.g., C10). Minute deposition in the regions modified with SAMs is believed to be due to the incomplete formation of a compact monolayer by µCP. This is consistent with the fact that zirconia thin films can be deposited on the CD substrate when the mountains are covered with C10 monolayers with an even better confinement effect. Finklea and co-workers38 have noted that the “blocking” properties of C18 SAMs are not perfect; that is, an electrode modified with such a monolayer resembles the behavior of microarray electrodes due to pinholes in the monolayer. The area fraction (0.09-4.0%) of pinholes was evaluated by oxide stripping measurements (i.e., gold oxide formation and stripping at pinholes). It was found that pinhole diameters are in the range of 0.1-10 µm and are separated by a distance of 1-100 µm. Zhao et al.39 have measured the surface density of defects in hexadecanethiolate SAMs on gold by amplifying the defects by chemical etching: they observed a minimum pit density of approximately 5 pits/ mm2. Losic et al.40 have found that SAMs prepared via µCP

J. Phys. Chem. C, Vol. 111, No. 38, 2007 14163 had significantly more pinholes and formed a less effective blocking layer than the solution-based counterparts, which are more robust and provide a better passivating layer.41 Diao et al.42 used electrochemical impedance spectroscopy and Fe(CN)63-/4- as redox probes to study how defects in octadecanethiolate SAMs (namely, pinholes and collapsed sites) change with adsorption time.42 They discovered that there is an initial rapid absorption step leading to loosely packed SAMs with a large number of pinholes; as adsorption time increases, the monolayer becomes more densely packed and the number of pinhole defects decreases. It is obvious that the presence of pinholes and defects in these monolayers contributes to the formation of zirconia thin films, which explains the minute deposition of zirconia on the C18 monolayer regions.34,38 In the case of CD substrates when the mountains were modified with C10 SAMs, we can still observe clear confinement of the film on these regions despite the blocking effects. This is due to the fact that both defects within the monolayer and the height difference of the two regions enhance the electrochemical deposition on the mountain regions. Nevertheless, accurately analyzing and quantifying the defects in SAMs proves to be quite challenging;38-40 it is even more difficult to quantitatively model their contribution to the electrochemical deposition of oxide thin films. Interesting results were obtained for flat gold surfaces patterned with both CH3- and OH-terminated SAMs in alternating strips of equal dimensions: when the chain length of the two molecules was of the same approximate length (C10 vs C11OH), no preferential confinement was observed. This may be due to the entire cathode being blocked by organic molecules, which allows only random deposition along the defects within the monolayer. However, different chain lengths (C6OH and C18) resulted in predominant confinement of the zirconia film into narrower strips (Figure 7). This remarkable finding shows that the morphology at even a molecular level has a significant influence over control of selective deposition. Such a unique nucleation/growth of oxide thin film may be due to the differences in (1) the terminal groups on the alkanethiols (OH is preferred vs CH3), and (2) the blocking effects created by the different chain lengths of the two thiols (C6OH favored in comparison with C18). Zamborini et al.43 have found that the extent of protection a SAM provides depends not only on chain length but also on terminal groups. It is not understood at this stage why the zirconia thin film was built up with a different periodicity with respect to the feature on the PDMS stamps; however, it is believed that the thiol molecules tend to rearrange and diffuse across the “boundaries” when the SAM is treated in another thiol solution. It is possible that at the interfaces of these two SAMs there is preferred diffusion of C18 to the C6OH region, resulting in the decreased width in the C6OH areas (where the cathodic deposition is favored). However, further studies to investigate the effect of defects in the different SAMs and provide a semiquantitative model are warranted. Electrochemical parameters play an important role in the selective deposition of the oxide thin films in all cases. Optimum conditions were found by scanning from +0.8 to -1.1 V (vs Ag|AgCl) in the present study. Scanning too negatively (past -1.1 V) resulted in little or no confinement as deposition occurred over the entire substrate. In addition to loss of selectivity at very negative potentials, H2 evolution begins to significantly disrupt deposition. Nevertheless, drawing a straightforward conclusion is difficult because in such systems many complex factors are at work simultaneously. Information concerning the local environment (pH, concentration gradients), the substrate morphology, the

14164 J. Phys. Chem. C, Vol. 111, No. 38, 2007 scale of the patterning, and the chemical nature of the surface must be considered before a more complete knowledge of the factors controlling film deposition is obtained. Further experiments are currently underway to answer some of these questions. However, it has been clearly demonstrated in this work that both surface morphology and chemical nature play a role to varying degrees in confining electrodeposition of oxide thin films by comparing gold cathodes with different surface morphologies and controlled chemical patterning. Conclusion This study has demonstrated that the selective deposition of metal oxide films (e.g., zirconia) is influenced by both the morphology and chemical properties of a surface. On CD-R gold substrates, it was shown that edge effects (resulting from surface morphology) dominated over blocking effects (resulting from chemical modifications with SAMs). When only the mountains were modified with SAMs, zirconia deposition was confined to the mountains despite these sites being blocked, which may be due to the effect of higher electrical fields along the edges and the presence of defects/pinholes in the monolayer. When the mountains and valleys were both modified (with different monolayers), no selectivity was observed but rather random agglomerates. In comparison, on flat gold surfaces the blocking effect of the SAMs contributed significantly to obtaining selective deposition of zirconia thin films on regions of bare gold. More significantly, when the substrate was patterned with alternating stripes of CH3- and OH-terminated monolayers of different chain lengths (C6OH vs C18), pronounced deposition was observed. Therefore, deposition is sensitive to both the chemical nature of a surface and morphology at a molecular level. Besides the surface morphology and chemical modification, the type of pattern and feature size of the zirconia film was dependent on scan rate, number of cycles, and terminal surface groups to a lesser extent. Acknowledgment. We thank the Natural Science and Engineering Research Council of Canada (NSERC) and Simon Fraser University for financial support. D.M.S. thanks Hanifa Jalali from Dr. Gates’ group for help with the microcontact printing experiments. References and Notes (1) Stevenson, K. J.; Hurtt, G. J.; Hupp, J. T. Electrochem. Solid-State Lett. 1999, 2, 175-177. (2) Walter, E. C.; Zach, M. P.; Favier, F.; Murray, B. J.; Inazu, K.; Hemminger, J. C.; Penner, R. M. ChemPhysChem 2003, 4, 131-138. (3) Zhitomirsky, I.; Gal-Or, L. Intermetallic and Ceramic Coatings; Marcel Dekker Inc.: New York, 1999. (4) Shacham, R.; Mandler, D.; Avnir, D. Chem.sEur. J. 2004, 10, 1936-1943. (5) Izaki, M.; Omi, T. J. Electrochem. Soc. 1997, 144, 1949-1952. (6) (a) Zhou, F.; Liu, Z. L.; Yu, B.; Chen, M.; Hao, J. C.; Liu, W. M.; Xue, Q. J. Surf. Sci. 2004, 561, 1-10. (b) Zhou, F.; Li, B.; Xu, T.; Chen, M.; Hao, J. C.; Liu, W. M. Sci. China Ser. B 2004, 47, 120-125. (7) Tao, Y. T.; Pandian, K.; Lee, W. C. Langmuir 1998, 14, 61586166. (8) Masuda, Y.; Jinbo, Y.; Yonezawa, T.; Koumoto, K. Chem. Mater. 2002, 14, 1236-1241. (9) Notsu, H.; Kubo, W.; Shitanda, I.; Tatsuma, T. J. Mater. Chem. 2005, 15, 1523-1527. (10) Zheng, H.; Rubner, M. F.; Hammond, P. T. Langmuir 2002, 18, 4505-4510. (11) Balaur, E.; Macak, J. M.; Taveira, L.; Schmuki, P. Electrochem. Commun. 2005, 7, 1066-1070.

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